Film treating apparatus



y 1961 A. SCHNEIDER ETAL 2,991,360

FILM TREATING APPARATUS Fild Nov. 21, 1955 INVEN TORS 4LFF0 Ja /V006,?

Patented July 4, 1961 United States Patent Otfice This/invention relatesto film'treating apparatus and relates more particularly to filmtreating apparatusfor subjecting a film to a high voltage electricaldischarge.

. fIt has been previously proposed to improve the printability ofpolyethylene films by subjecting said films to a high voltage electricaldischarge. In a preferred prior process, the film is passed between apair of electrodes having an extended surface area to which is applied ahigh alternating potential sufficient to produce a diffuse, coronadischarge between the electrodes. When this process is carried out on anormal commercial scale, treating films having a width of many feet andemploying large size treating apparatus, it is found that the treatmentis not fully uniform, with some areas being insufficiently treated forgood printability. To overcome certain of the drawbacks of this priorprocess, it is possible to increase the treating current employed sothat those areas receiving the minimum of treatment will possessadequate printability. However, when this is done many areas of the filmwill receive excessive treatment with the result that the surfacecharacteristics of the film, including the coefficient of friction, willbe altered in an undesirable manner.

It is an important object of this invention to provide a film treatingapparatus which will be free from the foregoing and other disadvantages.

A further object of this invention is to provide a film treatingapparatus for subjecting a film to a high voltage electrical dischargethat will produce a relatively uniform treatment throughout the entiretreated area.

Other objects of this invenion will be apparent from the followingdetailed description and claims.

It has now been found that the non-uniformity of treatment of the filmresults from variations in the treating current flowing throughdiiferent areas of the treating electrodes because of differingelectrical capacities between said areas. These differing capacities areproduced primarily by irregularities in the spacing between theelectrodes, which irregularities are difficult, to eliminate completelysince the treating electrodes have areas of many square feet.

According to the present invention, the effect of the differingcapacities is materially reduced by effectively inserting a capacitor inseries with the treating electrodes. When this is done, the percentagevariations of treating current between different areas of the treatingelectrodes will be appreciably reduced so that the uniformity oftreatment of treatment of the film will be improved. Under theseconditions, the treating current may be readily adjusted so that allareas of the film will receive adequate treatment to insureprintability, yet no areas of the film will be overtreated to such anextent as seriously to impair its properties.

The film treating apparatus of this invention comprises generally a pairof electrodes of extended surface area that are positioned so as to formbetween them an air gap of substantially uniform thickness. For example,the electrodes may comprise a pair of flat plates positioned parallel toone another and to which a high alternating potential is applied. Theelectrodes may also comprise a drum having a stator spaced from andconcentric therewith. The fihn being treated is passed between theelectrodes, spaced from the electrodes when .both surfaces of the filmare to be treated to improve its 'printability or in contact with one ofthe electrodes when only one surface of the film is to be treated. Theremay also be positionsd between the electrodes a sheet of dielectricmaterial to prevent an arc-over and damage to the film being treated inthe event that said film has pinholes or other weakspots therein.

A suitable source of high alternating potential is applied to theelectrodes to produce a corona discharge .therebetween. A capacitor ispositioned effectively in series with the treating electrodes and actsto make more uniform the distribution of the treating current betweenthe electrodes. The capacitor may be placed directly in series with thetreating electrodes. Alternatively, the capacitor may be placed inseries with the primary or low voltage winding of the high-voltagetransformer employed to supply the high alternating potential to thetreating electrodes. When a capacitor having a capacitance C is placedin series with the primary winding, it is equivalent to placing indirectly in series with the treating electrodes a capacitor having acapacitance where A is the transformer turns ratio. Placing thecapacitor in series with the primary or low voltage winding olfers theadvantage that low voltage capacitors are less expensive and more easilyinstalled than high voltage capacitors.

In general, it is desirable to use as small a series capacitor aspossible since this minimizes the influence of differing electricalcapacities between differing areas of the treating electrodes and tendsto make the film treatment more uniform. However, there is a practicallower limit to the size of the series capacitor since, as the size ofthis capacitor is reduced, the applied voltage must be raised to obtainthe desired treating current.

The drawing shows diagrammatically one embodiment of this invention.

Referring now to the drawing, the reference numeral 11 designates apolyethylene film which is guided by means of a roller 12 onto thesurface of a rotating drum 13 covered with a layer of dielectricmaterial 14 and provided with a slip ring 15 and a brush 16 forelectrical contact. Concentric with and spaced from the drum 13 is astator 17. A high alternating potential is applied between the drum 13and the stator '17 from a suitable source 18 through leads 19 and 21.Connected in the lead 19 in series with the treating electrode is acapacitor Through the use of this apparatus, it is readily possible toachieve an adequate treatment of all areas of the film for printabilitywithout, at the same time, overtreating any of said areas to such anextent as seriously to impair its other characteristics.

The following example is given to illustrate this invention further.

Example A film treating machine as shown in the drawing having a drumdiameter of 36 inches, a drum length of 71 inches, an air gap 0.125 inchthick and a layer of cellulose acetate dielectric 0.02 inch thick hasinserted directly in series with the treating electrodes a capacitorhaving a capacitance of 0.2 microfarad. The uniformity of the treatmentof a polyethylene film passing through the apparatus is materiallyimproved.

It is to be understood that the foregoing detailed description is givenmerely by way of illustration and that many variations may be madetherein without departing from the spirit of our invention.

Having described our invention what we desire to secure by LettersPatent is:

1. Film treating apparatus comprising a pair of spaced electrodes ofextended surface area positioned so that the spacing between theelectrodes is substantially uniform, means for applying a highalternating potential difference to said electrodes to produce adischarge therebetween, means for passing a film between said electrodeswith said electrodes on opposite sides of said film, and a capacitorelfectively in series with said electrodes.

2. Film treating apparatus comprisinga drum, a stator spaced from andconcentric with said drum and forming with said drum a pair of spacedelectrodes, means for applying a high alternating potential differenceto said electrodes to produce a discharge therebetween, means forpassing a film between said electrodes with said electrodes on oppositesides of said film, and a capacitor effectively in series with saidelectrodes.

3. Film treating apparatus comprising a drum, a stator spaced from andconcentric with said drum and forming with said drum a pair of spacedelectrodes, a sheet of dielectric material between said electrodes,means for 4 producing a difiuse corona discharge between said electrodesand including means for applying a high alternating potential ditferenceto said electrodes, means for passing a film between said electrodeswith said electrodes on opposite sides of said film, and a capacitoreffectively in series with said electrodes.

References Cited in the file of this patent UNITED STATES PATENTS

